Means for welding thermoplastic sheets



L. 1.. YOUNG 2,609,482

MEANS FOR WELDING THERMOPLASTIC SHEETS a Sheets-Shet 1 Sept. 2, 1952Filed July 23, 1946 ll W FIG. I

Lam, L. Youn Sept. 2, 1952 L. YOUNG 2,609,482

MEANS FOR WELDING THERMOPLASTIC SHEETS Filed July 25, 1946 v:5Sheets-Sheet 2 PRIOR ART 2/, F \G. 2 F

PRIOR ART as, FIG. 3 K 3:

I v PR IOR ART +3 4/ 42 H6. 4 E31;

PRIOR ART 5/ 52 FIG. 5

Larr L. Young L. L. YOUNG MEANS FOR WELDING THERMOPLASTIC SHEETS 3Sheets-Sheet 3 Sept. 2, 1952 Filed July 23, 1946 INVENTOR. Larr L. Youngthermoplastic material.

Patented Sept. 2, 1952 MEANS FOR WELDING THERMOPLASTIC SHEETS Larry L.Young, New York, N. Y., assignor, by

mesne assignments, to Padevco, Inc., Washington, D. 0., a corporation ofDelaware Application July 23, 1946, Serial No. 685,647

' 9 Claims. (01. 219-47) This invention relates to the art of weldingthermoplastic materials using high frequency dielectric heating.

Thermoplastic materials in the form of sheets or plates have beenlaminated or welded together by pressing them between platens which canbe heated and then cooled. In this case the thermw plastic sandwichbecomes heated to plasticity by the flow of heat from the platens. Thematerial is then cooled by the flow of heat back into the cooledplatens. In this conventional method, the amount of heat exchanged inone heating and cooling cycle may be great because the heat capacity ofthe platen may be much greater than that of the thermoplastic sheet. Asa result, conventional methods are wasteful of power, and a cycle ofoperation is objectionably slow.

In dielectric heating by high frequency displacement current, heat isgenerated within the This phenomenon makes it feasible to obtaintemperature distribution, heating times, efiiciency, and other specificresults which are impossible to obtain wtlh conventional means andmethods.

The most simple arrangement is to press the sandwich sheets togetherbetween metal electrodes which restrain the material and produce therequired high frequency field in the sheets. With such simple means,however, heat can flow by conduction into the metal electrodes at a ratewhich prevents the sheet interfaces from reaching a sufficiently hightemperature until the electrodes also reach an elevated temperature. Therate of heat generation may be increased by increasing the potentialgradient through the sheets or by increasing the alternating frequency.There are practical limitations of increasing fretations encountered inthe prior art.

The prime object is to affect a welding of the sheets.

Another object is to weld together a specific area of the interfacebetween two sheets without causing an overall warping or distort on ofhe sheets.

Another object is to weld an area of the-interface of two sheets withoutcausing a local-deformation or extrusion at the weld which might beobjectionable in appearance or which might result in a weakenedstructure.

Another object is to provide a method of welding thermoplastic sheetswhich israpid and convenient.

Another object is to provide means which are simple, inexpensive, andwhich will withstand much usage.

Another object is to provide a. means" and method which is economical inthe use of power.

Another object is to provide a means and -method of welding togetherthin sheets of thermoplastic material.

Another object is to provide a means and method capable of weldingtogether sheets of thermoplastic material which, because of theirthinness, low loss factor or high temperature .of

plasticity, have been difficult or impossible .to weld by similar meansand methods heretofore attempted.

The invention will be fully understood-by reference to the followingdescription and drawings in which:

Figure 1 shows a schematicarrangement of electrodes pressed againstthermoplastic sheets and coupled to a source of high-frequency power.

Figure 2 shows an arrangment of conventional electrodes in position toweld two thermoplastic sheets. v

Figure 3 shows one kind of unsatisfactory weld obtained withconventional electrodes characterized by electrical and thermalbreakdown'at electrode edge.

Figure 4 shows another form of unsatisfactory weld obtained withconventional electrodes characterized by local deformation andextrusion.

Figure 5 shows an unsatisfactory weld obtained with conventionalelectrodes in which deformation and extrusion combined with a bendingand separation of the sheets has occurred.

Figure 6 shows a pair of electrodes according to my invention with thecross-section of one shown in detail.

- electric flux and regions of temperature rise.

. a thin thermoplastic sheet of a thick sheet.

Figure 11 shows a ring-shaped electrode constructed in accordance withthe invention.

In Figure 1, l and 2 are sheets of thermoplastic material pressedbetween electrodes 3 and 4. 5 and 6 are electrical insulators whichinsulate the electrodes from the fixed members'8 and 9. A spring member1 maintains pressure on the thermoplastic sheets I and 2. An oscillatorin provided with controls ll, [2, and I3, which determine frequency ,fand voltage e applied for a short period of time t to the electrodesthrough coupling I4 and leads l5 and l6. l1 represents a ground for thelower electrode. l8is a starting button.

The method of using these means-to make a weld in the thermoplasticsheets is asfollows:

With the sheets I and 2 in placeand'controls ll, l2, and I3 properlyset, the starting button 18.

is pressed. A pulse of predetermined frequency and voltage is,thereupon. automatically applied to the electrodes.

The powder is automatically ltnrnedoifafter a shortperiodioftime tdetermined'by the setting of. l 3.. Compressing the spring member 1andmoving the welded sheets completes one. welding cycle.

124 to which a pulse of high frequency potential is applied as in themethod decribed above. With low power and pulses of shortduration, thereis negligible effect on the thermoplastic and little tendencyto weld.

Figure 3 illustrates an unsatisfactory weld showing charred perforations3|, bubbles3'2, and scorched spots 33-a-t the line where the plastic'isheetsextend from the electrodes.

' Figure 4 shows a weld which was'obtained at a lower power but withpower applied for a long time. The sheetsshow objectionable plasticdeformation 4l and extrusion 42. Tl'ie interface may be poorly bonded at43.

l' igurefishows in addition to the faults shown inFigure 4, a bending 51and spreading 52 of the sheet beyond the weld line.

' The applicanthasascertained through analysis and experiment that heatflow in and near the weld area of the thermoplastic sheets and in theelectrodes which indirectly produce the highfre-.quencydisplacementcurrents in the sheetsand which press against thethermoplastic sheets is an important factor. This heat flow depends onthe thermal. 'conducivityofthe materials of the electrodeandfthethermoplastic sheets, on the ge- -ometry of the electrode andsheets, on the tem .perature distribution in theelectrode and sheets.

at any instant. Stated more completely: In

' .order'to producea certain favorabletemperature distribution in thethermoplastic sheets which will enable one to obtain the above-named objeetives, itisnecessaryto employ electrodes which yield proper heat flowand electrical functioning, and

t a-pply a certainpotential of certain high fre- 'quency andfora certainperiodof timeto these electrodes.

Referring to Figured, thermoplastic sheets Bl Y and. 62 are shown inposition to be welded between two electrodes. A sectional view of theupper electrode shows a good electrical conductor 63 such as copper,silver or Kovar; The rounded end .ofthe conductor Mhas bonded toita'dielectric material such. as glass'which separates the con- 1 "Poorthermal conductor. High dielectric constant.

12 3 f1. Low poweriactor at high frequency.

5. Low loss factor at high frequency.

6. High dielectric strength.

'7. High mechanical strength.

8,. Must withstand sudden changesin' temperature such as between-0 and250 Centigrade without damage.

9. Non-porous.

10. Smooth finished.

The specific values-for one embodiment'of'applicants invention "were asfollows:

Plastic materialCellulose.acetate .010 thick. Electrodes--Glass-Kovar(circular section) ,-1Kovar 0.30" diameter; glass 0.450 diameter, 0.020thickness on flat end. Seal area-0.30 diameter. Frequency- 910megacycles persecond. Potential on electrodes-300.0 volts. Time0.20second. I Instantaneous power dissipatedBO watts.

The weldingoperation is as: follows:

The electrodes press the sheets together-and determine. theconfigurationof alternating high frequency flux through thethermoplastic. The electrodes being in contact with the sheets. alsodetermine theheat flow conditions.

The potentialapplied to the electrodes. determines-theflux density andpotential gradient in the thermoplastic. and therefore affects the'volumetric rate of heatgeneration. The frequency :of oscillation of thecurrents also affectsthe rate of heating. Other factors aifecting theheatingare the nature and condition of the thermoplastic material.

The temperature rise at any; particular'point in the thermoplastic.sandwich .dependson rate-of heat generation, on heat flow, on. thenature-and condition of the thermoplastic material. and on the heatingtime. I

Using the means and. method shown, a.high degree of control over thesefactors is obtained.

In Figure 7 lines of electricfiux ll, 12 and 13 between the electrodesare represented. Inthe arrangement shown, the flux density at avpoint inthe thermoplastic such as 14. will be greater thanin the region (5 under.theedge of the electrode. It isestablished. that the volumetric: rateof. heat generation is. proportional. .to the square of the electricflux density.

An important feature of the electrode isthe thermal insulating functionof the dielectric part of the electrode underlying the endof'thelelectrical conductor above and below the region ".14. In thearrangement shown, the thermal insulation of the region 14 is high andthet hermal insulation of theadjacent region. '15 is not much higher. 1

An important advantage of'the electrode arrangement shownis that bysurrounding theend of the electrical conductor with dielectric mate- '15so thatthis portion, which does not reach a temperature sufficient forplasticity, remains tightly wedged between the electrodes and serves asa pressure dam to confine the plastic region M. In this mannerconsiderable pressure can be obtained in region 14 which pressure isnecessary to affect a high quality bond. Thus, the regions 15 incombination with the electrodes form a mold, in effect, for the region14, and any thermoplastic which can be molded at suitable pressure andtemperatures can be welded in the manner shown. 4 l

A feature of this method of welding is that the thermoplastic sheets donot reach a high temperature at the edge of the electrodes; welds cantherefore be obtained without distorting or warping the sheets andwithout causing a local deformation or extrusion at or near the weld.

An important feature of this method is based on the fact that the'heat'flow phenomenon is critically dependent on the element of time.Heat is generated in the thermoplastic region 14 at a rate sufficient tocause the temperature of this region to rise rapidly in spite of theflow of heat energy from this region. This requires a high potentialgradient through the thermoplastic, but the weld may be accomplished inonly a small fraction of a second.

An important feature of this method is that by producing largerdisplacement currents for a limited time, the heat developed raises thetemperature of the region 14 efficiently and only a small fraction ofthe heat developed is wasted. Therefore, the amount of heat energy whichflows into the electrodes during a welding cycle is small and, theelectrodes remain cool. The structure of the electrodes is such thatheat flows out of the region '54 rapidly only after the weldingtemperature has been reached and the high frequency power has been shutoff. This permits extremely rapid welding. Since the heat loss is low,only low power required and small and inexpensive hi h frequencygenerating equipment is required.

As an example of the versatility of the method, reference is made toFigure 8 in which BI is a thin thermoplastic sheet, 82 is a thickthermoplastic sheet, 83 is an electrode, and 84 is a thick electricallyconducting sheet of large surface area. 85 represents a high frequencycoupling, 86 a ground connection, and 81 a ground connection to thelarge conductor representing means for putting the electrode at a highfrequency potential relative to the large conductor when high frequencypower is supplied to the coupling 85.

The object of this arrangement is to weld the region of the interfacebetween the thin and thick sheet without causing any of the faults ofdeformation or extrusion.

Figure 9 shows a variation in the detail construction of the electrodes.The cross-section in a plane perpendicular to the face of the electrodeshows a thin metallic conducting shell in the form of a U, 9 l, thebottom of which may be perforated 6 92. Dielectric material is bonded tothe conductor as shown to form a face 93, a projecting edge 94, and aninternally supporting structure 95.

This construction is utilized to provide a greater control of heat flowand an improved mechanical structure.

Figure 10 shows a cross-section of the electrode of Figure 9 taken alongthe line A-A in Figure 9. 91 is the shell, 32 are the perforations inthe shell, and 94 is the dielectric forming the projecting edge of theelectrode.

Figure H shows means for making a weld of large area in the form of aclosed ring in one welding cycle.

HI is a metal channel and H2 is the dielectric face edge.-- H3 and H4are the conducting channel and dielectric revealed in cross-section.

Whereas the description and drawings have in the main describedelectrodes which may be used for spot-welding a small area of theinterface of two sheets and which may be used to forrn'overlapping weldsto form a line weld, I do not wish to be so restricted, the scope of myinvention being defined by the claims.

I claim:

1. An electrode for high frequency dielectric heating comprising ametallic member having a flat side, a thin coating of low lossdielectric material having a low thermal conductivity on said side, anda bead of said material of arcuate cross section round the periphery ofsaid side and integral with said coating, said coating having athickness of less than .05"inch,'and said head a thickness at leastseveral times that of said coating.

2. High frequency dielectric heatin apparatus including a high frequencygenerator, a pair of electrodes connected to the high frequencygemerator, one of said electrodes comprising a metallic member having aflat side, a thin coating of low loss dielectric material having alow'thermal conductivity on said side, and a head of said material ofarcuate cross section round the periphery of said side and integral withsaid coating, said coating having a thickness of less than .65 inch, andsaid head a thickness at least several times that of said coating.

3. High frequency dielectric heating apparatus for welding sheets ofthermoplastic material including a high frequency generator, a pair ofelectrodes juxtaposed on opposite sides of the thermoplastic sheets,means for holding said electrodes against said thermoplastic sheetsunder pressure, each of saidelectrodes comprising a metallic memberhaving a flat side, a thin coat ing of low loss dielectric materialhaving a low thermal conductivity on said side, and a bead of saidmaterial of arcuate cross section round the periphery of said side andintegral with said coating, said coating having a thickness of less than.05 inch, and said head a thickness at least several times that of saidcoating.

4. An electrode for high frequency dielectric heating comprising ahollow metallic cylinder, one end of said cylinder having perforations,a low loss dielectric material having low thermal conductivity forming acontinuous, plane, thin coatin over the perforated end of the metalliccylinder and extending through said perforations to within saidcylinder.

5. The electrode described in claim 4 in which the metallic cylinder isKovar and the dielectric material is glass.

6. High frequency heating apparatus for thin thermoplastic sheetcomprising, a source of high to said source of high. frequency -vo1tage,at

rjieast one .of -.said electrodes comprisin ,a meta JET-body :h n a hatface, ;a dielectric material having :low thermal conductivity and l w idelec- 1 tric loss in response to said voltage forming a heating over:said face said coating having a thickness of less than..0 :inch, a headof said 1 cdielectricxmaterial of arcuate cross sectional/er- :aging atleast two times said thickness about the periphery of said face andintegral with said acoating, means :for compressing said .sheetbewtweenzsaid electrodes, means for apply ng sai high frequency voltageto said electrodes in a short pulse, :said p l endurin for a ime o yqsufficientqto eiiect softening of said sheet Without substantialheating of said at least one elec- 'it1t0deIhy1fl0w.Qf'heatfromsaidsheet toisaid at least one electrode via .s.aid coating, and means for-maintaini ressure of said electrodes son said sheets after terminationof said ulse until ,-h e a t ,has flowed from said sheet to-said one ofasaid-electrodes via said coating in suificientquantity to enablesolidifying of said sheet.

7. An electrode structure for high frequency dielectric-heating,comprising, a hollow elect-rode having a flat electrode surface, saidelectrode surface having at least one aperture therein, said electrodesurface fabricated of Kovar, and a and. low dielectric lossesin responseto ,said volt- ,agehmeans for compressing said sheets between saidfaces, means for applying said high frequency voltage to, saidelectrodesin a short ulse, ;:said pulse enduring for a time onlysufiici'ent to effectfiow of said sheets without substantial heat- 1.ing-ofsaid electrodesby flow of heat from said sheets to saidelectrodes via said coating, and means for maintainin pressure of saidelectrodes ,onsaid sheets after termination of said pulse until heat hasflowed from said sheet to said .one of isaidelectrodes via said coatingin suf- Lficient quantity to enable solidifying of said -sheet.

l9. rHigh frequency heating apparatus for ther- ;,moplasticsheets,rcomprising, a source of high lffrequency voltage, a pair ofelectrodes connected fr quen y v taae' o said el ct des in ashdrt Pulses id pulse enduring. f r a t e onl litficient to effect softening ofsaid s lr tets withoiit sub an al heat n f aid at leas one q S ele o esb flow o h athen wids t aid a v least en Q s electro es and m an for maitaining p ess e of s id e ss trqdes o sa dshee s .aiter te mination 9isaid pu s'fiei heat flawed irqm sa d sh et a saide leas on of saidelectrodes i sufficient ant t enable ooling of said she thel spinn ngen- .nerature.

LBEE BENQESQJEEP The following references are of record -in the file ofthis patent:

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